Multimedia devices become increasingly more popular. As the technology advances, the devices shrink and consequently so do their respective displays. Nowadays people watch television broadcast in their cell phone having an average of less than 10 cm2 screens. Thus, the need for a light, removable, reliable and high resolution projector has never been bigger. One type of the devices that is being developed in the recent year is the personal projectors in which fast micro mirrors are used in controlling the path of a laser beam thereby providing the required images.
One of the difficulties in the design and fabrication of the fast mirrors is the requirement to deliver both large tilting angles and larger resonance frequencies which may be improved by reducing the mass of the mirror. However, reducing the mirror's mass can only be done by thinning the mirror (assuming that both the material of which it is made and the beam diameter are kept constant), leads to a new problem—development of dynamic deformations.
When the mirror vibrates, it experiences deformations resulting from its undeformed flat surface due to its inertial force and torque from the torsion bars referred to hereinafter as dynamic deformation, which disrupt the optical beam wavefront and damage the achievable resolution.
Some solutions have been suggested in the past, among which is the common solution of adding support structures to the mirror. These support structures add mass to the mirror and by careful design, they increase the mirror's rigidness thereby reducing the dynamic deformation, without adding too much to its mass.
US 2003039021 discloses a MEMS mirror and method for fabricating the mirror. The mirror has a plurality of structures that operatively rotate around a support structure. The mirror is fabricated, such that the silicon components are separated from a glass structure having electrodes to prevent shorting of the electrodes to the mirror.
US 2004190110 describes a MEMS device having a deformable segmented mirror. The mirror includes a plurality of movable segments supported on a substrate using spring vertices, each vertex having a fixed plate and one or more springs. Due to the fixed plates, motion of each movable segment is substantially decoupled from that of the adjacent segments, which makes the shape of the segmented mirror relatively easy to control.
US 2004245888 discloses a MEMS device having a deformable mirror and includes a deformable membrane supporting a plurality of light-reflecting segments that form the deformable mirror. The actuator(s) which is configured to apply torque to the side of the membrane, is used to deform the membrane. Membrane deformation causes the segments to change orientation and thereby change the shape of the mirror, reduce it deformation and improve the resolution.
US 2007019280 describes MEMS devices that comprise a conductive movable layer spaced apart from a conductive fixed layer by a gap, and supported by rigid support structures, or rivets, overlying depressions in the conductive movable layer, or by posts underlying depressions in the conductive movable layer. It also describes cases where portions of the rivet structures extend through the movable layer and contact underlying layers.
In “Electrostatic 1D Micro Scanner with Vertical Combs for HD Resolution Display” by Jin-Woo Cho et al. Proc. Of SPIE vol. 6466, pp. 64660B-1 to 64660B-12, an electrostatic one dimensional scanning mirror is described in which a vertical comb drive was used to tilt the micro mirror, and a rib was used to minimize the moment of inertia and maximize the tilting angle of the mirror. The rib described was patterned on the upper silicon layer before making the SOI wafer.